1. Field of the Invention
The present invention relates to a mechanism for maintaining a moving web of sheet material, such as paper, in a predetermined position relative to a stationary guide. The invention has specific application as part of a photographic printing apparatus in which images from photographic film are sequentially projected and printed on photographic paper moving in a continuous web through the apparatus. It is not limited to that environment, however.
2. Description of the Prior Art
In any apparatus in which paper or other sheet material is fed in a continuous stream, such as from a feed roll, the moving web has a tendency to wander side to side from its desired path of travel. In a photographic printer, it is essential that the web of photographic print paper be held precisely to its proper path through the various process stations to insure that all process steps are correctly performed and that the final print is properly centered.
A typical prior art web tracking apparatus is shown and described in Research Disclosure of March 1980, item #19160 disclosed by J. E. Morse. The Morse publication describes a roller which is pressed against and rolled on the surface of a web which is being pulled over a stationary support or platen. The roller axis is canted or skewed by some small angle relative to the perpendicular to the web path. As the paper moves, in addition to rolling action, there is continual lateral gripping or scuffing action between the soft compliant roller and the moving web due to the angular skew. This continual lateral gripping action forces the web laterally against an edge guide plate mounted along side the web path. With respect to such apparatus the skew angle is critical. If the skew angle is too large the gripping action both continual and macroscopic in magnitude can be harmful to delicate surfaces such as a photosensitive paper. If the skew angle is too small the desired tracking action is not obtained.
The skew angle must be periodically readjusted in such prior art web tracking apparatus due to the variability of various influences, such as wear and aging of the compliant roller, changes in coefficient of friction and variations in the contact force of the roller.